Solve for $z$ : $z - 6 = 15$
Add $6$ to both sides: $ \begin{eqnarray} \\ z - 6 &=& 15 \\ \\ {+6} && {+6} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-6 + z} &\hphantom{=}& \hphantom{15} \\ z &=& 15 {+ 6} \\ \end{eqnarray} $ Simplify. $ z = 21$